Measurement of automotive exhaust gases for hydrocarbons, CO, and NO.sub.x has been common in recent years as Federal standards of increasing severity have been applied. measurement of particulates in exhaust emissions has been of interest, although no standards for these emissions have been established. Automotive particulate emissions in general may be defined as any material, other than unbound water, which condenses at 90.degree.F. into particles larger than a small molecule, but smaller than 500 microns in diameter. Concerns over these emissions are related to the potential effects on human health which may result from the formation of these minute particles in the atmosphere. Such particles could among other things possibly function as sites for condensed phase chemical reactions to assist in the production of smog. Larger particles, notably lead particles, produced from atomotive exhaust settle rapidly and do not remain suspended for long periods as do the smaller particles toward which the present invention is directed. Earlier investigations were particularly concerned with the production of lead compounds resulting from the use of tetraethyl lead in motor gasoline. More recently, particulate emissions from automobiles operating with catalylic converters which are likely to be required to meet Federal standards have been of concern.
Typical of earlier work was that reported by J. B. Moran and O. J. Manary, Interim Report PB196783, "Effect of Fuel Additives on the Chemical and Physical Characteristics of Particle Emissions in Automotive Exhausts", NAPCA, July, 1970, and K. Habibi, "Automotive Particulate Emissions and their Control", SAE Paper 710638, Oct. 24, 1970, Midland, Michigan. This earlier work was primarily related to lead compounds and used equipment which is not suitable for measuring the smaller particles of present concern, especially where these particles must be collected over a rather short test period with the extremely irregular operation of the vehicle typical of a programmed trip cycle. Accordingly, there has been a need for measurement of the small particulates in a manner which would give rapid and reproducible results suitable for use with the standard Federal test cycles. The present invention is a method and apparatus which accomplishes this objective.
Making such measurements is a more difficult task than might be at first thought. In a typical test cycle, the automobile exhaust gases may range from 20 standard cubic feet per minute up to 200 standard cubic feet per minute, constantly changing as the vehicle accelerates or deaccelerates according to the standard program. In order to insure uniformity of results, it is desired to maintain a constant temperature filtration of the exhaust, which is incompatible with the marked variations in flow rate which are typical of a test cycle. This temperature has been selected on a preliminary basis to be 90.degree.F., which introduces the possibility that condensation of water in the exhaust will occur, thus scrubbing a portion of the particulate matter from the exhaust. Since only small quantities of particulates are produced during a typical cycle of perhaps 20 to 40 minutes, it is essential that all of these particles be caught and retained in the filter in order to give accurate and repeatable results. It is neither practical nor possible to adopt the technique of some of the earlier investigators, which involved disassembling the apparatus and cleaning it internally in order to recover the last traces of particles which have been produced. Since repetitive tests are run, it is important that equipment not require disassembly and that all of the particles produced be trapped by the filter for analysis. What is required then is that particles leaving the tailpipe of an automobile or other vehicle must be thoroughly mixed with dilution air, which will be discussed more fully later, and the particles must not be lost between the mixing point and the filter assembly, either due to settling or wall impactions. It will be clear that some residence time is necessary to provide suitable mixing and in any case is unavoidable in construction of the apparatus. In addition, the system cannot operate at such a temperature that condensation of water from the exhaust will occur. If it does, the scrubbing action produced will remove particulate matter, giving irregular and obviously erroneous results. Also, particle agglomeration would be likely to occur during the condensation process, making measurement of the distribution of particulate matter impossible. Only a portion of the total exhaust is actually filtered, making the mixing process particularly significant if the sample is to correctly reflect the composition of the exhaust gas. Another problem not immediately evident is that since cooling to approximately 90.degree.F. is required, this cooling must be done in such a manner that thermophoretic deposition will not occur. Once the exhaust gases have been cooled and mixed, a uniform and reproducible sample must be taken in order to determine the quantity, chemical composition, and size of the particles. Isokinetic sampling which is known broadly in the solids sampling art is utilized in the present invention. However, inasmuch as the quantity of particulate matter is small and the time over which it is collected is relatively short, the isokinetic sampling system itself is subject to some of the problems which have already been discussed with regard to particulate losses. Accordingly, the isokinetic sampling system itself must prevent particulate losses. The particulate sampling system should be compatible with the constant volume sampling system which is used in the measurement of other exhaust gaseous emissions; that is, both gaseous and particulate emissions should be measured at the same time over any typical test cycle simultaneously. This constant volume sampling system has been referred to earlier in connection with the dilution of exhaust gas with air. Since a vehicle during a typical test cycle will produce a wide variation in exhaust gas flow rates, and presumably in compositions as well, it is necessary to sample the exhaust gases at varying rates corresponding to their production. The constant volume sampling procedure provides for dilution of the exhaust gases with an amount of air which is sufficient to produce a constant total volume of exhaust gas and air. Once a constant volume is obtained, then sampling can be done at a constant rate no matter what exhaust gas rate is being produced and if the exhaust gases and air are completely mixed, the sample will be proportional to the amount of exhaust gas being produced at any given time in the test cycle. It will be appreciated that the constant volume system could be useful for particulate sampling as well. The introduction of air is beneficial to particulate sampling in that condensation of exhaust gases can be partially prevented. However, it is not eliminated; it has been found that at points in a typical test cycle the 90.degree.F. temperature which is desirable for filtering particles will result in condensation of exhaust gases even though diluted by ambient air. Of course, on extremely humid days the problem will be more severe than in dry weather. Temperature of ambient air also varies widely so that a complicating factor is introduced into the particulate sampling since a constant temperature must always be available at the filter, which collects the particulate matter. In view of the difficulties and problems which have been outlined above, it will be appreciated that the accurate sampling of particulate emissions in automotive exhaust represents a difficult problem, but it has been solved by the method and apparatus of the present invention.